Morphine

Morphine is the standard opioid with which others are compared and remains a valuable drug for the treatment of acute, severe pain.Peak effect after IV bolus is 15 min. Duration of action is between 2 and 3 hours. Both liver and kidney function are responsible for morphine elimination. The liver mainly metabolises it. One of the principal metabolites, morphine 6-glucuronide (M6G), is also a potent opioid agonist and may accumulate in renal failure.

Uses
Relief of severe pain
To facilitate mechanical ventilation
Acute left ventricular failure – by relieving anxiety and producing vasodilatation

Contraindications
Airway obstruction
Pain caused by biliary colic

Administration
• IV bolus: 2.5 mg every 15 min PRN

• IV infusion rate: 1–5 mg/h
Dilute in glucose 5% or sodium chloride 0.9% Stop or reduce infusion each day and restart when first signs of discomfort appear. Failure to assess daily will result in overdosage and difficulty in weaning patient from ventilation

• If the patient is conscious the best method is to give an infusion pump they can control (PCAS): 50 mg made up to 50 ml with sodium chloride 0.9%; IV bolus: 1 mg; lockout: 3–10 min

How not to use morphine
In combination with an opioid partial agonist, e.g. buprenorphine (antagonises opioid effects)

Adverse effects
Respiratory depression and apnoea
Hypotension and tachycardia
Nausea and vomiting
Delayed gastric emptying
Reduced intestinal mobility
Biliary spasm
Constipation
Urinary retention
Histamine release
Tolerance
Pulmonary oedema

Cautions
Enhanced and prolonged effect when used in patients with renal failure, the elderly and in patients with hypovolaemia and hypothermia. Enhanced sedative and respiratory depression from interaction with:
• benzodiazepines
• antidepressants
• anti-psychotics

Head injury and neurosurgical patients (may exacerbate ↑ ICP as a result of ↑ PaCO2)

Organ failure
CNS: sedative effects increased
Respiratory: respiratory depression
Hepatic: can precipitate coma
Renal: increased cerebral sensitivity. M6G accumulates

Renal replacement therapy
CVVH dialysed dose as in CC 10–20 ml/min, i.e. use smaller than usual dose, e.g. 2.5–5 mg. HD dialysed dose as in CC < 10 ml/min, i.e. use smaller doses, e.g. 1.25–2.5 mg and extended dosing intervals. PD not dialysable, dose as per HD. Active metabolite M6G accumulates in renal failure.Titrate to response, such as pain/sedation scores.